I just had a couple more suggestions for your awesome script.
I think it would be really cool if maybe there can be an option for both the arms and legs to turn off the maintain volume feature.
What I mean is that right now, if I stretch out the arms or legs in IK with stretch on, or I use the scaleX in when in FK mode the limbs become thinner (as they should to maintain volume). But sometimes i need to fake some stuff in a shot and this requires me to make the characters arms and or legs longer while still maintain their normal volume. But with the way its setup now, the arms and legs get thin when you stretch them out.
Also, if this option were to be added, when maintain volume is off, when in FK could there be scaleX scaleY and scaleZ for both the upper and lower limbs seperately so we can get even more customization if needed.
One last thing would to be given more control on the scale of the hand. Right now when you scale it, it scales evenly. It would be nice to have individual scales so that for example during a smear frame, you could make the hand really long and thin.
So I dont know if any of this is even possible but I just thought I would throw a few more suggestions your way.